ctxcancel.go

package ctxcancel

import "context"

// CtxCancel run f in background and return result
// onCancel called if ctx cancelled before f return result
// it called after f finish
// return f result of ctx.Err() (if ctx cancelled)
func CtxCancel(ctx context.Context, f func() error, onCancel func()) error {
	fResult := make(chan error, 1)
	go func() {
		err := f()

		fResult <- err
		close(fResult)
	}()

	select {
	case <-ctx.Done():
		// pass
	case err := <-fResult:
		return err
	}

	if onCancel != nil {
		go func() {
			<-fResult
			onCancel()
		}()
	}

	return ctx.Err()
}